I. Cross Reference to Related Applications
II. Field of the Invention
This invention relates generally to an improved cardiac rhythm management apparatus, and more particularly to an implantable cardiac stimulating device incorporating means for optimizing the A-V interval or other pacing parameters without the need for an additional sensor.
III. Discussion of the Prior Art
In the "Discussion of the Prior Art" section of the above-identified companion application, there is set out at length a discussion as to how the prior art in the field of implantable cardiac stimulating devices has not addressed a closed-loop adaptive system for adjusting the A-V interval between the occurrence of an atrial depolarization signal (natural or paced) and the generation of a succeeding ventricular stimulating pulse in such a way that the heart's cardiac function is optimized, especially in treating patients suffering from congestive heart failure (CHF). In accordance with the teachings of that application, a patient suffering from CHF has implanted in his/her body a cardiac stimulator and lead arrangement which allows for the sensing of atrial and ventricular activity along with a variable frequency pulse generator for stimulating at least one ventricular chamber, that pulse generator being controlled by a microprogrammable controller. In addition to the usual lead assembly used conventionally for sensing atrial and ventricular depolarization signals, a further sensor is utilized for measuring cardiac function and providing information relative thereto to the microprogrammable controller. That controller then operates to adjust the A-V interval in incremental steps until reaching a predetermined A-V delay limit value. For each of the incremental steps, the cardiac function sensor allows the system to determine whether a given incremental change in the A-V interval results in improved cardiac function. Ultimately, the A-V interval is set to the greatest length which is less than or equal to the limit value that yields maximal cardiac function.
The present invention differs from the device described in the aforereferenced companion application in that it obviates the need for an additional sensor, i.e., the sensor used to assess cardiac function.
Those skilled in the art will appreciate that the body has several mechanisms designed to adjust to metabolic demand by modifying cardiac output and, ultimately, oxygenation of tissue. The cardiac output may be increased or decreased by adjustment in sinus rate or stroke volume. Recognizing this fact, it should be possible to monitor metabolic demand by monitoring the sinus rate or the stroke volume or both. The present invention takes advantage of these mechanisms to optimize cardiac function in paced patients.
In a situation where the pacing rate is under sinus control, for example, in VDD pacing, any modification in a pacing parameter which decreases cardiac performance (and thus cardiac output) will, after a period of adjustment, result in an increased heart rate to regain the original cardiac output. Conversely, any change which improves cardiac performance will result in a decreased heart rate. Thus, the minimum, steady-state heart rate achieved following an incremental adjustment of a performance related pacing parameter, e.g., atrial-ventricular interval, will correspond to the optimum value of that parameter. It is, therefore, possible to optimize these parameters simply by monitoring the heart rate during the adjustment process, noting the value at which the minimum sinus rate occurs and setting the pacemaker to this parameter value at the conclusion of the procedure.
Before the above approach can be used to effect optimization of cardiac function, it is necessary to guarantee that the patient is in a stable hemodynamic condition at the time that heart rate is being measured following an incremental adjustment in the A-V interval. If the underlying condition is changing during the adjustment procedure, such as when the patient is exercising or under emotional stress, the heart rate will no longer be simply related to changes in the parameter being optimized and there can be no guarantee that the optimization procedure will converge to the correct value.
It is accordingly an object of the invention to provide an apparatus and method for optimizing cardiac function utilizing a cardiac stimulator that does not require a further sensor over and above the conventional sensing and stimulating electrodes on a conventional pacing lead.
Another object of the invention is to provide a method and apparatus for optimizing cardiac function in a patient suffering from cardiac disfunction, such as CHF, by setting the A-V delay interval of the implantable stimulating device to a value which corresponds to the lowest heart rate obtained during a series of measurements taken over a predetermined period of time and at times when the patient is found to be in a stable hemodynamic condition.